The overall structure of the individual TransmissionModes can be printed to the REPL using the following code:

julia> using EnergyModelsGeography
julia> const EMG = EnergyModelsGeography
julia> using AbstractTrees
julia> AbstractTrees.children(x::Type) = subtypes(x)

julia> print_tree(EMG.TransmissionMode)
├─ PipeMode
│  ├─ PipeLinepackSimple
│  └─ PipeSimple
├─ RefDynamic
└─ RefStatic

The leaf TransmissionModes of the above type hierarchy tree are composite types, while the inner vertices are abstract types. The individual TransmissionMode and their fields are explained in the public library.